The Diablo preceded the Super Diablo. I'm not positive when production started but I have an ad from the July '59 issue of the National Bowhunter for "Howatt's Improved Hunters" showing the Diablo.
The Diablo in the ad does not have a leather grip so if your bow does, I assume that it would at least be earlier than '59. The ad bow has a stripe in the riser as well.
The serial number on the early bows ('60 or so and earlier) aren't a lot of help in dating, the decals, logos, riser shape and woods will be the most help. One thing that may be present is a number stamped in the riser about 3/8" tall identifying the bowyer. Damon Howatt was bowyer #1 in the early days prior to the sale of his company.
Please post pics when it arrives.
Brad